I just got back from our local Z club event at the dyno. Results are attached. My engine is as follows:

- N42 block with dished pistons (~80k miles)

- E31 head (stock valves, ~10k on rebuild)

- MSA 6:1 header w/2.5" exhaust

- Pertronix electronic ignition

- SU round tops with ZTherapy bodies, and .099 nozzles and SM needles

I live at 6500 ft elevation although the shop was about 1000 feet lower. They say the results are corrected for altitude, temperature, etc.

I tried 2 other runs. On one I turned the mixture nuts in (leaner) 1 full turn. On the other put the mixture back, and advanced the timing 4 degrees. In both cases peak HP dropped ~3 hp. So I figure I'm not too far off the optimum. Or am I?

So my question is what you seasoned engine tuners think. Is there any "easy" power to be had through tuning? I'm not going to spend any more $$ on the engine for a while and instead begin to concentrate on brakes and suspension.

Pretty interesting. Notice how the torque peaked right before 2.5k and stayed the same.

I don't think your gonna gain much more than that without $$$. But with the basic set up you have I'd be happy.

Thanks

Ed, I noticed the same thing. In fact, my first "butt-o-meter" impressions after swapping this motor in place of the old L24 was how much stronger it was at low revs. Looking at this chart I was surprised that there wasn't some increase in torque with revs. I suppose the cam/valve train would need to be changed to improve things, and the $$ would be significant.

The one thing I notice is that you are running lean at the top end starting around 4K, cam or carbs? I wonder what it would have been if you would have richened the carbs instead of leaning them a turn and left the timing the same??

As the operator explained it to me, the bottom chart is the air to fuel ratio. As the number gets smaller it means there is less air (richer). So we read this as I'm getting richer at the top end. Hence we tried turning the nozzles in a turn. He overlayed the two runs and they were virtually identical. And the engine didn't idle well at the leaner setting either.

Yes that is exactly right. The engine is not breathing enough for those needles at high rpm 11:1 a/f is very rich. Of course, torque and hp will always cross at 5252 rpm, so the fact that the hp starts dropping off at that point indicates not enough cam. So yes you have come to the right conclusion. Bigger valves and higher lift cam would turn your Datsun into a monster.

I would be happy with the car as is though. Just from the chart I can tell that it launches really easily and is a pleasure to drive even if it lacks a bit on the top end.

Another question: I've experimented in the past with advancing the cam timing via the different positions on the sprocket to take up slack. I've noticed it extends the power peak to higher rpms at the expense of torque at lower revs. My question is if there is any INCREASE in power, or does it the same power peak just occur at a HIGHER rpm? Unfortunately I don't have access to the dyno again to measure this accurately. I figure someone might have experimented with this on a dyno with the stock cam.

Any increase in power will be minimal wihout an increase in lift. Advancing the cam timing does indeed only move the power band

A question for the tuning gurus.

Is there a limitation on what the standard round tops can do? For a street car such as 240zman's car is it unnecessary to go for bigger su's or triple webers until you are talking BIG HP?

thanks for any replies.

ill rub some salt in the wound with my mates L20b dyno results..

its got twin 45mm dcoe webers.

stage 3 crow cam

genie extractors 4into 1 design

2inch exhaust.

it kinda makes your l28 results look like wet fish.

Originally posted by abas

Is there a limitation on what the standard round tops can do? For a street car such as 240zman's car is it unnecessary to go for bigger su's or triple webers until you are talking BIG HP?

Abas, I'm not a tuning guru but at the dyno day there were two L28s with mods like mine, with the addition of a mild cam. One had triple webers, the other triple mikunis. Both put out about the same HP as mine, and both were much too rich at high revs. I suspect the richness hurt their power as they should do better, but the point is I think that both were over-carburated.

There's a story I've seen mentioned many times about a guy up in the northwest who runs the quarter mile in the 12's using roundtops. I can't verify that, but suspect that it takes some very serious engine mods before the stock carbs can't keep up.

Thanks for the info 240zMan :classic:

This is what I had hoped for, as I am not going for big HP on mine, and hopefully the standard SU's will do for me.